1. Field of the Invention
The present invention relates to a device for detecting the degree of deterioration of a catalyst.
2. Description of the Related Art
In an engine, a catalyst is normally arranged in the exhaust passage to purify the exhaust gas. Such a catalyst, for example, a three way catalyst has an O.sub.2 storage function such that it absorbs and stores excess oxygen existing in the exhaust gas when the air-fuel ratio of the air-fuel mixture fed into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, i.e., when the air-fuel mixture becomes lean, and that the catalyst releases oxygen when the air-fuel ratio of the air-fuel mixture fed into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, i.e., when the air-fuel mixture becomes rich. Accordingly, where the air-fuel ratio is alternately changed on the rich side and the lean side of the stoichiometric air-fuel ratio, since excess oxygen is absorbed and stored in the three way catalyst due to the O.sub.2 storage function thereof when the air-fuel mixture becomes lean, NO.sub.x is reduced. Conversely, when the air-fuel mixture becomes rich, since the oxygen which has been absorbed and stored in the three way catalyst is released therefrom, HC and CO are oxidized. Accordingly, Nox, HC and CO can be purified at the same time.
Therefore, in a conventional engine, an air-fuel ratio detector for detecting the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is arranged in the exhaust passage upstream of the catalyst. When the air-fuel mixture becomes lean, the amount of fuel to be fed is increased and, when the air-fuel mixture becomes rich, the amount of fuel to be fed is reduced. As a result, the air-fuel ratio is alternately changed on the rich side and the lean side of the stoichiometric air-fuel ratio, and thus the amount of NOx, HC and CO is reduced at the same time.
However, if the three way catalyst deteriorates, the purifying rate of exhaust gas by the three way catalyst is reduced. In this case, since the exhaust gas is purified due to the O.sub.2 storage function of the three way catalyst, if the O.sub.2 storage function is weakened, the three way catalyst deteriorates. Accordingly, if the fact that the O.sub.2 storage function is weakened can be detected, the deterioration of the three way catalyst can be detected.
Therefore, in a known engine, a second air-fuel ratio sensor is additionally arranged in the exhaust passage downstream of the three way catalyst. The air-fuel mixture fed into the engine cylinder is changed over from a rich mixture to a lean mixture after the air-fuel mixture is maintained at, for example, a rich mixture for a fixed time. After the changeover of the air-fuel mixture from the rich mixture to the lean mixture, the air-fuel ratio detected by the second air-fuel ratio sensor is changed from the rich side to the lean side of the stoichiometric air-fuel ratio with a time interval. In this case, if this time interval is shorter than a predetermined time, it is determined that the three way catalyst is deteriorated. In this engine, the deterioration of the three way catalyst is detected by noting the fact that, if the O.sub.2 storage function is weakened, when the air-fuel mixture fed into the engine cylinder is changed over, for example, from a rich mixture to a lean mixture, the length of time during which the second air-fuel ratio thereafter continues to produce a rich signal indicating that the air-fuel ratio is on the rich side becomes short.
Now, the ability of the three way catalyst for reducing NO.sub.x and oxidizing CO and HC decreases as the actual amount of oxygen which can be stored in the three way catalyst is reduced. Accordingly, the actual amount of oxygen which can be stored in the three way catalyst correctly represents the degree of deterioration of the three way catalyst. Therefore, to correctly detect the degree of deterioration of the three way catalyst, it is necessary to correctly detect the actual amount of oxygen which can be stored in the three way catalyst. In this case, the actual amount of oxygen which can be stored in the three way catalyst cannot be found based on only the length of time during which the second air-fuel ratio continues to produce the rich signal in the above-mentioned engine, and therefore, in the above-mentioned engine, a problem arises in that it is impossible to correctly detect the degree of deterioration of the three way catalyst.